This invention relates to collision detection in communication systems.
Local area networks (LANs) connect personal and mini-computers with each other and with other shared resources, for example, printers, file servers, and mainframe computers. Separate LANs can be joined via bridges to form larger linked systems of computers and shared resources.
One of the most popular standards for construction and operation of LANs is the IEEE 802.3 Protocol for Local Area Networks, popularly called the Ethernet standard. The 802.3 standard prescribes a carrier sense multiple access with collision detection (CSMA/CD) protocol for medium access control (MAC), where "medium" refers to a communication medium. The 802.3 CSMA/CD standard allows a number of LAN devices to communicate within a shared communication medium, without central message arbitration. Each LAN device, when sending a message, listens to the communication medium at the same time. If another LAN device also begins sending a message, the first LAN device can sense the multiple access and thereby detect a "collision" between the signals of the two messages. For example, in an Ethernet LAN using coaxial cable as its transmission medium, and employing a digital baseband method of signaling, a collision occurs when the first LAN device starts to send a signal but simultaneously detects a signal coming from a second LAN device within the transmission medium. The first LAN device declares a "collision", and that device (and any other LAN devices also attempting to send messages) implements a standard back-off protocol. The back-off protocol typically requires the LAN device to wait increasingly longer random periods of time after each collision before attempting to resend its message.
The 802.3 CSMA/CD standard has also been implemented in non-metallic transmission systems, such as optical fiber and spread-spectrum radiofrequency communications networks. The 802.3 CSMA/CD standard may be applied to any broadcast medium, so long as the medium can support a method for collision detection. For coaxial cable media (as above), one collision detection method measures the energy density at the transceiver, since signal levels of all transceivers measured anywhere along the transmission line are similar.
Infrared communication techniques for LAN systems are attractive because they offer a number of benefits, including low transceiver costs, low power consumption, high device reliability, high biosafety, low installation costs, and little required cabling. Existing infrared transmission standards (for example, IRDA94) provide for data transmission point-to-point, single access protocols. These are typically half-duplex, utilize amplitude modulation (AM), and do not provide any form of collision detection. Instead, these standards typically rely on collision avoidance. A true multiaccess infrared LAN requires a method of collision detection for high utilization of available IR bandwidth. Furthermore, a useful method of IR collision detection would allow implementing the full 802.3 CSMA/CD protocol, permitting an IR multiaccess LAN to integrate seamlessly with other conventional LAN systems and devices.